I generally do not purchase the back until I am finished with the top. The reason is I seldom purchase all of the fabric for the top. I use from stash and depending on how it all shifts out, I may want to choose a different color or fabric for the back than what I would have originally, perhaps emphasizing a different color that I originally considered. I almost always use the backing fabric as my binding. I do still have a very few cuts left in my stash that are backing size for larger than baby quilts, but very few. Good question.